Thursday, December 5, 2013

Blast from the past: Modern Phytophthora infestans infections

By NK Between the years of 1845 and 1852 approximately one million people died and a million more emigrated from the country of Ireland (1). The main cause for this death and desertion was the Irish potato famine. Although multiple events lead to this devastation, the most apparent cause was the destruction of the country’s potato crop by late potato blight (see figure 1). This disease is caused by the pathogen Phytophthora infestans. P. infestans is a fungus like eukaryote that is included in the class of oomycota (2). The pathogen infects potatoes and tomatoes and then causes damages to the foliage and the root systems of the plants. P. infestans is capable of invading the potato itself, causing massive damage and rendering the potato useless as a food source.

Many people consider P. infestans to be a relic of the past; a pathogen of a different
era. The cold hard truth is that P. infestans is still alive and causing
destruction. The pathogen currently causes over $6.7 billion dollars annually in potato crop damages (3). The reality behind this pathogen is that there appears to be few ways to stop it. P. infestans has a massive
genome that is rapidly evolving beyond human control. Recent findings point to the rediscovery of
the A2 mating type and the evolution of effector proteins as reasons for this pathogen’s resurgence and resilience (4,5,6).

P. infestans asexual cycle is important for host invasion. The sexual cycle allows P. infestans to develop diversity and increase the genetic variability of the organism. P. infestans has two mating types or “sexes”, A1 and A2 (4). A1 is the most frequently found mating type while A2 mating types are considered rare (4). Despite having sexes the organism has the ability to reproduce asexually. The asexual cycle involves hyphal growth, zoospore (mobile), and sporangia germination (5). The zoospores are the most important
component in attacking the plant host. The zoospores are expelled from the sporangium into the plant where they
can swim and infect various parts of that plant (see figure 2) P. infestans only induces sexual structures when it is in the presence of their opposite mating type, which allows for the formation of oopspores (4,5). Before 1970 most of the P. infestans populations in Asia, Europe, and Africa were from one clonal population (4). It was also found that this clonal population only contained the A1 mating type. The A2 mating type was only found to be common in certain areas in central Mexico (4)(see figure 3). After 1970 the A2 mating type was beginning to be found outside of Mexico. The spread of the A2 is cause for concern as demonstrated by an outbreak of P. infestans that occurred in the Toluca Valley of Mexico in 1988 (6). A study performed by J. Matuszak found that this outbreak contained a population of P. infestans that possessed both mating types. It was also found that this population had a much greater genetic diversity then the other P. infestans populations (6). The study highlighted the importance of sexual mating and genetic diversity increasing the ability of these pathogens to infect potatoes. By the 1980s the A2 mating type was now common to many European and Asian countries. The distribution of this mating type likely resulted from international trading of potatoes from Mexico to foreign countries. As stated before sexual reproduction allowed for and continues to allow for the formation of more diverse pathogens. With both sexual types becoming more prevalent it is only a matter of time before another outbreak, similar to the Toluca Valley incident, occurs somewhere else. The sexual cycle also provides an advantage for the pathogen that most people have overlooked. P. infestans is limited during its asexual cycle as the zoospores and hyphal stages cannot survive colder conditions. The oospores produced during the sexual cycle could provide the pathogen with a means of surviving colder environments (4). This development of more pathogenic and resistant oospores would cripple potato farmers as the soil could potentially be useless for years.

Farmers have attempted to use resistant potato crops in order to control P. infestans infections. Recently the pathogen has started to gain the upper hand with resistant potatoes by the use of effector proteins (2). Effector proteins have many functions for pathogenic organisms. In P. infestans their main role involves
suppressing the host’s immune system. This is true in potatoes which have an immune system. In fact potatoes have complex immune systems with multiple genes that help regulate their immune system (2). Studies have shown that a potato gene, called RB, is crucial in providing resistance to P. infestans infections (2). A recent
study found that a certain class of effector proteins becomes more active in the presence of this RB gene. The
effector proteins used in the study were proteins that have a conserved motif called RXLR. An RXRL effector protein variants called IPI-O were highly variable and abundant in P. infestans(2). The most interesting protein was IPI-O4. The experiment used plants expressing and not expressing the RB gene and infected them with IPI-O4 producing P. infestans(2). The result was the IPI-O4 caused the pathogen to become more aggressive in attacking the plant that contained the RB gene. The result suggests that IPI-O isolates have the ability to overcome RB resistant plants. The IPI-O isolates were found to be incredible varied and multiple isolates other than IPI-O4 conveyed this increased aggressiveness (2). This large diversity of IPI-O within the P. infestans indicates
that the pathogen might be overcoming resistant potatoes defenses. The amount of IPI-O4 variants was low in the population tested. It is possible that these variants could be introduced into new potato crops similar to the
migration of the A2 mating type. If this variant becomes more pronounced within the P. infestans population then farmers could see massive devastation to their thought to be resistant potato crop. Potatoes are the third most consumed food crop and are vital for millions of people
throughout the world. (7) The pathogen P. infestans is a real threat to this plant. Many people have ignored this pathogen simply because it is associated with the Irish potato famine and has been pushed into the history books. The pathogen currently causes billions of dollars in damages worldwide. The warning signs are increasing for an even larger impact. The increase in genetic variability due to A2 mating type prevalence and revelation of IPI-O4 effector proteins overcoming resistance is cause for concern. More research is needed to help develop effective control methods against this pathogen or we could see a worldwide epidemic.

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